Abstract This study attempted to find clinical variables evaluated at age 2 that would predict mental retardation (MR, IQ/cognition-adaptation developmental quotient [C-A DQ] < 70) at age 5 in 57 children with pervasive developmental disorder (PDD). About two-thirds of subjects had MR at both initial and outcome evaluations. The C-A DQ at initial evaluation was significantly lower in mentally retarded PDD (MRPDD) than in high-functioning (IQ ≥ 70) PDD (HFPDD). MRPDD changed less than HFPDD in IQ/C-A DQ between ages 2 and 5. The C-A DQ at age 2 was a potent predictor for MR at age 5 and the total score and three item scores of Childhood Autism Rating Scale-Tokyo Version evaluated at age 2 were also useful in predicting MR at age 5.